1. Field of the Invention
The present invention relates to an electrolytic enrichment method for heavy water, in which heavy water is enriched by electrolysis with an alkaline water electrolysis cell.
2. Description of the Related Art
Heavy water is water containing many isotopic water molecules with large mass numbers and having higher specific gravity than general water. Heavy water has physical and chemical properties slightly different from those of general water. General water is referred to as “light water” relative to heavy water. Heavy water contains hydrogen isotopes such as deuterium (D, 2H) and tritium (T, 3H), and oxygen isotopes 17O and 18O.
In the field of determination of safety of an atomic power plant, prediction of crustal movement, measurement of a hot-spring groundwater system, or the like, analysis of deuterium (D, 2H) and tritium (T, 3H) in natural water is becoming important. A tritium concentration is a very low level, and thus electrolytic enrichment is generally conducted for improving measurement accuracy. A generally known electrolytic enrichment method for heavy water includes preparing a sample solution in which an electrolyte is dissolved, and electrolyzing the solution using flat plates disposed to face each other. Water contained in an electrolyte contains HOD and HOT in addition to H2O, and these are decomposed into hydrogen and oxygen according to usual water electrolysis, but H2O is decomposed in preference to decomposition of HOD and HOT by the isotope effect to increase the concentrations of deuterium and tritium in the electrolyte, resulting in enrichment. In this electrolysis, nickel is used as an anode, and steel, iron, or nickel is used as a cathode, and these electrodes are washed and used for electrolysis under current-carrying conditions in a glass container which contains sample water prepared by adding diluted sodium hydroxide as a support salt to an aqueous solution containing heavy water. Heavy water is generally enriched by continuing electrolysis with a current density of 1 to 10 A/dm2 until a liquid amount becomes 1/10 or less while the solution temperature is kept at 5° C. or less in order to prevent water evaporation due to the heat generated.
That is, electrolytic enrichment uses the property that tritium water is less electrolyzed than light hydrogen water. A method for electrolysis using metal electrodes inserted into an alkaline aqueous solution has already been frequently studied and publicly standardized in a manual as a standard method. This method includes one-stage enrichment of tritium concentrations. However, in practice, a general method for electrolytic enrichment has some problems. The problems lie in complicated experiment operations, a tritium enrichment rate limited to the upper limit of an electrolyte concentration, a danger of explosion due to the occurrence of mixed gas of hydrogen and oxygen, much time required for electrolysis, and unsuitableness for large-capacity treatment.
Since a technique is considered in view of one-step separation and capture of a dilute inclusion, the problems are due to trouble with handling an aqueous alkaline electrolyte solution, difficulty in separating gases generated on both electrodes, difficulty in increasing an electrolysis current due to the generation of bubbles on a metal surface, etc., which are mainly caused in a general electrolytic method of an aqueous alkaline solution.
On the other hand, a water electrolytic method recently attracting attention is a water electrolytic method (hereinafter referred to as “SPE water electrolysis”) using a solid polymer electrolyte (hereinafter referred to as “SPE”). In the early 1970s, US General Electric Company first applied fuel cell technology to the SPE water electrolysis in such a manner that an electrolysis portion having a structure including a SPE membrane held between porous metal electrodes is immersed in pure water, and electrolysis is performed only by passing a current, generating decomposed gas from the porous electrodes. The SPE is one type of cation exchange resins and has a polymer chain to which sulfonic acid groups contributing to ionic conduction are chemically bonded. When a current is passed between both electrodes, water is decomposed, and oxygen gas is generated on an anode, generating hydrogen ions. The hydrogen ions move through the sulfonic acid groups in the SPE, reach the cathode, and receive electrons to generate hydrogen gas. The SPE is apparently maintained in a solid state without being changed. When the SPE is used for tritium electrolytic enrichment, the advantages below can be expected as compared with usual methods.
1) Distilled water can be directly decomposed. That is, dissolution, neutralization, and removal of an electrolyte, which are essential for an electrolytic method of an alkaline aqueous solution, are not required, and a volume reduction factor of sample water is basically unlimited.
2) An electrode surface is not covered with bubbles, and thus electrolysis can be performed with a large current, thereby shortening the electrolysis time.
3) Since hydrogen gas and oxygen gas are generated separately on both sides of the SPE membrane, the gases can be easily treated. This is far safer than a usual method including handling explosive mixed gas.
Also, the applicant has proposed an electrolytic enrichment method for heavy water by SPE water electrolysis in Japanese Unexamined Patent Application Publication Nos. 8-26703 (U.S. Pat. No. 3,406,390) and 8-323154 (U.S. Pat. No. 3,977,446) and Tritium Electrolytic Enrichment using Solid Polymer Electrolyte (RADIOISOTOPES, Vol. 45, No. 5, May 1996 (issued by Japan Radioisotope Association).
However, the method proposed in these documents can be applied to an analysis apparatus and small-scale enrichment treatment, but is unsuitable for large-scale treatment. No current flows through the electrolyte because the electrolyte used is pure water, and thus the solid polymer membrane used as a component must be strongly crimped with the anode and the cathode under surface pressure corresponding to 20 to 30 Kg/cm2. Therefore, members of an electrolysis cell are required to have high strength, and in view of economy and operationality, it is unrealistic to secure a large reaction area of 1 m2 or more, thereby undesirably increasing the equipment cost for electrolytic enrichment and fractionation of raw material water containing a large amount of heavy water.